1. Field of the Invention
This invention relates to metal chelate complexes for forming oxidation catalysts, and more particularly, to long-lived macrocyclic oxidation catalysts capable of catalyzing demanding oxidations with peroxidic and related primary oxidants.
2. Description of the Invention Background
While transition metal-based systems provide the major source of oxidants in both chemistry and biology, oxidation chemistry is much better developed in the latter area, i.e., many difficult selective oxidation reactions that are accomplished in biological processes have not been achieved in homogeneous synthetic systems. This difference is more glaring for oxidation chemistry than for any other major branch of reaction chemistry. Thus, compared with reduction chemistry or carbon-carbon bond forming chemistry, oxidation chemistry is still severely limited in the number and quality of the available technologies for stoichiometric or catalytic processes.
The relative dearth of good homogeneous oxidation systems and catalysts is believed to be due to oxidative degradation. Complexes of high oxidation state middle and later transition metal ions, analogous to those that function as active intermediates in-numerous enzymatic oxidations, have been difficult to attain synthetically because of the tendency of such complexes to quickly degrade their ligands.
In Collins, T. J., "Designing Ligands for Oxidizing Complexes," Accounts of Chemical Research, 279, Vol. 27, No. 9 (1994), synthetic metal-based oxidants are conceptually separated into two classes, metalloredox-active oxidants and metallotemplate oxidants. In metalloredox-active systems, the oxidizing moiety contains the metal ion which is in direct contact with the ligands. Consequently, these systems are limited by the small supply of ligands that are compatible with oxidizing metal ions. Metallotemplate oxidants are not limited in such a way because the oxidizing entity is more remote from the metal ion, but metallotemplate systems are useful only for mild as opposed to rigorous oxidations that require highly reactive metalloxidants. The metal ion oxidants in oxygenase enzymes often catalyze rigorous oxidations such as the methane monooxygenase reaction, i.e., the oxidation of methane to methanol with oxygen as the primary oxidant. The roles of the metallo-oxidants in such enzymes are of the metalloredox-active type. Thus, a key to moving this spectacular enzymatic chemistry into man-made systems lies in conquering the challenge of developing robust ligand systems that can tolerate extremely strongly oxidizing metal ions of the atom-abstractor type.
In the Accounts article, Collins describes a design-oriented approach to formation of ligands and metal chelate complexes that are resistant to oxidative degradation. The Accounts article highlights a set of rules for attaining ligand systems that are inert to oxidative degradation. Several diamido-N-diphenoxido acyclic and tetraamido-N macrocyclic ligands, developed to be resistant to oxidative degradation, are also illustrated in the Accounts article, as are middle and later transition metal complexes where the metal ions are in rare or unprecedented high oxidation states attainable by employing the macrocyclic ligands.
While being sufficient to allow preparation of the described rare high valent ions in stable form, including strong electron-transfer oxidants, the set of rules of the Accounts article is incomplete for achieving the goal of encapsulating an especially powerful metal-oxo oxidant similar to those found in monooxygenase enzymes such that the oxidant has a sufficient lifetime to carry out bi-molecular oxidations. Attainment of such a goal had to wait to the developments in ligand design described herein.